1. Field of the Invention
The present invention relates to a static pressure gas bearing for use as a bearing for rotating parts in precision machinery and the like.
2. Description of the Prior Art
Conventionally, a static pressure gas bearing is widely used to support a rotating member such as a rotating bearing and the like which is incorporated into a precision machinery or the like and rotates at high speed, such that the rotating member is supported by the force of a compressed gas such as air.
This type of conventionally known static pressure gas bearing as depicted in FIG. 12 and FIG. 13 is disclosed in Japanese Patent Publication of Examined Application No. 45-37683.
In FIG. 12, a rotating shaft 3 is inserted to the inside of a bearing member 1 of which the inner peripheral surface forms a cylindrical bearing surface 2. Provided in the bearing surface 2 are a plurality of indented sections 4a, 4b, 5a, 5b which are formed at the four positions in the top, bottom, left and right respectively of the cylindrical bearing surface 2. The rotating shaft 3 is then supported in a non-contact state at the inside surface of the bearing member 1 by a compressed gas supplied to the indented sections 4a, 4b, 5a, 5b.
A compressed gas supply source such as a compressor (not shown) is communicated through a first throttling control valve 6 with the indented sections 4a, 4b, which are the upper and lower sections of the four indented sections 4a, 4b, 5a, 5b. This compressed gas supply source is also communicated with the left and right indented sections 5a, 5b through a second throttling control valve 7.
The volume and pressure of the compressed gas fed to the upper and lower indented sections 4a, 4b and to the left and right indented sections 5a, 5b are regulated, respectively, through the first and second throttling control valves 6, 7, so that the concentricity of the bearing surface 2 with the outer peripheral surface of the rotating shaft 3 is maintained. The first and second throttling control valves 6, 7 of this type have, for example, a configuration such as illustrated in FIG. 13.
The first throttling control valve 6 is here described for purposes of explanation, but this description may be applied equally well to the second throttling control valve 7. Therefore, the following description includes the second throttling control valve 7 and its members in parenthesis.
In FIG. 13, a first port 9 provided at the center of one surface of a housing 8 which forms the first throttling control valve 6 (the second throttling control valve 7) is communicated with the indented section 4a (5a) by a first supply tube 10. Also, a second port 11, provided at the center of the other surface of the housing 8 is communicated with the indented section 4b (5b) by a second supply tube 12. In addition, a diaphragm 15 is provided in the middle section of the housing 8. The diaphragm 15 divides the inside of the housing 8 into a first chamber 13 on the side of the first port 9 and a second chamber 14 on the side of the second port 11. A compressed gas is fed from the compressed gas supply source (not shown) into the first and second chambers 13, 14 as shown by an arrow.
At one part of the inner surface of the housing 8, the sections surrounding the openings of the first and second ports 9, 11 project inward or toward the diaphragm 15, each extending over the entire periphery of the opening. As a result, a first throttling passage 16 is formed on one side of the diaphragm 15 in the section between the first port 9 and the first annular chamber 13, and a second throttling passage 17 is formed on the other side of the diaphragm 15 between the second port 11 and the second annular chamber 14.
In the case where the outer peripheral surface of the rotating shaft 3 and the bearing surface 2 are not concentric, caused by the displacement of the rotating shaft 3, the volume and pressure of compressed gas entering the indented sections 4a, 4b and 5a, 5b are suitably regulated by the action of the first and second throttling control valves 6, 7, so that the outer peripheral surface of the rotating shaft 3 and the bearing surface 2 become concentric.
For example, when the rotating shaft 3 is displaced downward in FIG. 13, the clearance dimension of a bearing gap 18 between the outer peripheral surface of the rotating shaft 3 and the bearing surface 2 is reduced in the lower portion closer to the indented section 4a (5a) and increased in the upper portion closer to the indented section 4b (5b). As a result of this dimensional change, the pressure is increased within the lower indented section 4b (5b) and decreased within the upper indented section 4a (5a). Therefore, the pressure within the second port 11 communicated with the lower indented section 4b (5b) through the second supply tube 12 increases, and the pressure within the first port 9 communicated with indented section 4a (5a) through the first supply tube 10 decreases.
As a result, the diaphragm 15 which separates the first and second ports 9, 11 is displaced upward, so that the second throttling passage 17 widens and the first throttling passage 16 narrows. This causes the volume and pressure of the compressed gas entering the lower indented section 4b (5b) to increase, and the volume and pressure of the compressed gas entering the upper indented section 4a (5a) to decrease. The rotating shaft 3 is then pressed in the upward direction in FIG. 13, so that the displacement of the rotating shaft 3 is corrected.
However, the following drawbacks are inherent in a conventional static pressure gas bearing with this type of structure and action.
Specifically, with this conventional structure, the indented sections 4a, 4b, 5a and 5b for supplying compressed gas to the bearing gap 18 are comprised of relatively large depressions. Because of this, self-induced vibration is readily produced by compressed gas entering the indented sections 4a, 4b, 5a and 5b. When this self-induced vibration occurs, the operation of the precision machinery and the like in which the static pressure gas bearing is incorporated becomes unstable.
In particular, as shown in FIG. 12 and FIG. 13, when the first and second supply tubes 10, 12 for communication between the first and second throttling control valves 6, 7 and the indented sections 4a, 4b, 5a and 5b are long, the flow control response is poor and the abovementioned self-induced vibration is readily produced.